WASHINGTON, Oct 7 (Reuters) - He has been stereotyped as the Obama administration's gaffe-prone sideshow.

But Vice President Joe Biden also is a veteran debater who was in the U.S. Senate for a quarter century and is perhaps the Democratic White House's most passionate defender of the working class.

Now, with his debate against Republican vice presidential nominee Paul Ryan coming up on Thursday, Biden is under pressure to help President Barack Obama's campaign recapture the momentum it enjoyed before Obama was outmaneuvered by Republican rival Mitt Romney last week in the first of their three debates.

That debate trimmed Obama's lead in the polls and raised the stakes for the lone vice presidential debate, which also will be a national debut of sorts for Ryan, the Wisconsin congressman and budget specialist.

With the Romney-Ryan campaign energized, Democrats are in the surprising position of relying on Biden, a perpetual political wild card, to fire up their campaign. Democrats did get a boost in Friday's jobs report, which showed the nation's unemployment rate dipped below 8 percent last month for the first time since January 2009, the month Obama took office.

"This is not about changing minds. This is about changing the momentum," Democratic strategist Chris Kofinis said. "The vice president is going to have to be very aggressive in undercutting Republican arguments. He can't allow Ryan to control the debate the way Romney controlled his debate."

An aggressive Romney went after Obama in their debate, offering what appeared to be new positions - misrepresentations, Democrats claimed later - on taxes, healthcare and other issues.

Obama, strangely passive, left many of Romney's assertions unanswered. It will be up to Biden, far more comfortable than Obama in the role of aggressor, to fill in the blanks on what Democrats say are Romney's shifting positions.

Biden could be well suited to the role.

He was a sharper performer than Obama when they debated as presidential candidates during the 2008 Democratic primaries. The former Delaware senator's blunt-talking style makes him a particularly effective communicator with blue-collar voters.

"Biden has a chance to undo some of the damage from the first debate," said David Steinberg, a debate coach and political communications specialist at the University of Miami.

"The vice president's biggest job will be as a fact checker," he said. "He can come in and say, 'Well, this is what Governor Romney said last Wednesday, and this is why it's wrong.'"

Biden, campaigning in Iowa on Thursday, said he had been studying Republican campaign positions and promised to hold Ryan accountable while sticking to the facts.

"I don't want to say anything in the debate that's not completely accurate," Biden told reporters. "I just want to make sure that when I say those things I don't have the congressman say, 'No, no, no, I don't have that position,' or 'That's not the governor's position.'"

The next presidential debate - the second of three - will not be until Oct. 16, leaving Thursday's vice presidential showdown in Danville, Kentucky, as the next major item on the campaign calendar in the race to the Nov. 6 election.

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Vice presidential debates rarely play a role in deciding a White House race, but Romney's decisive win in the first presidential debate has cranked up interest in Thursday's encounter.

Biden, the former chairman of the Senate Foreign Relations and Judiciary committees, fared well in his face-off against Republican Sarah Palin in the 2008 vice presidential debate and proved to be a strong debater during his failed bid for president four years ago.

But he also has a reputation for gaffes, including his recent remark that the middle class has been "buried for the last four years" - the span of Obama's presidency - by a bad economy.

Biden was referring to the fallout from the recession that began under Republican President George W. Bush, but Romney and Ryan pounced on Biden's comment, saying that even Obama's running mate was acknowledging that the president had fallen short in overseeing the economy.

Last spring, Biden forced Obama's hand on endorsing same-sex marriage when the vice president declared his support for it during a television interview.

Biden is portrayed at times as a political buffoon, but his defenders say the mistakes are evidence of a straight-talking style that makes him a hit with many voters - and that can play well in a debate setting.

"Biden has got something going for him in a debate, which is a sense of humor," said Alan Schroeder of Northeastern University's Alan Schroeder, who has written a history of presidential debates.

"Both Romney and Obama are humor-challenged, and Paul Ryan has not shown himself to be a barrel of laughs, so Biden has a chance to bring some theatricality and some show-business energy to the debate that will make it more interesting," Schroeder said.

Ryan, the chairman of the House Budget Committee, is popular with conservatives for his plan to slash government spending and create a "voucher" system for the Medicare healthcare program for seniors. Democrats say that a voucher system could leave some seniors having to pay for much more of their medical costs.

Ryan is an unknown quantity in a debate setting, with his only previous experience coming in a few low-profile congressional encounters in his native Wisconsin.

"This is the big game and Ryan is playing on a stage that he's never had experience with, so he'll be under a great deal of pressure, too," Schroeder said.

A Reuters/Ipsos online poll of 2,367 voters this week found both men have work to do on their image, with "very unfavorable" being the most popular view of each. Biden scored better than Ryan, 39 percent to 33 percent, on the question of who was more qualified to take over as president.

Ryan and Biden have taken time off the campaign trail to prepare for Thursday's debate.

Democratic Rep. Chris Van Hollen has played Ryan in mock debates with Biden; former U.S. Solicitor General Ted Olson has played Biden in sessions with Ryan.

Van Hollen said Obama's performance in last week's debate had not changed the Democratic strategy heading into the vice presidential encounter.

"The focus is on the choice the American people face in this election," he said.

Even if the vice presidential debate is typically less meaningful than the presidential encounters, neither campaign can afford a slip-up before an expected television audience in the tens of millions.

"Every inch matters in this campaign," Republican strategist Ron Bonjean said. "The polls are going to tighten up, and you don't know what Biden you are going to get."

But Biden's reputation for mistakes could lower expectations for him.

"It would be nice to be Biden," Steinberg said. "He's got a wider range of stuff he can say and people go, 'Oh, that's just Biden.' If Ryan said it, they will be looking at him through a microscope."